It is difficult to evaluate the impact of new medical technologies unless the patients being compared are adequately described. The challenge of precise description is especially difficult in acutely ill hospitalized patients for whom traditional diagnostic categories do not accurately reflect severity of illness. Therefore, the goal of this project is to develop, test and validate a classification system for acutely ill hospitalized patients. We will use as our prototype APACHE (Acute Physiology and Chronic Health Evaluation), a classification system that uses simple, objective, and readily available physiologic measurements combined with an expert scoring system. In preliminary tests on 600 patients, APACHE classification was reliable across different observers and applicable in a wide variety of patients admitted to Intensive Care Units. The final APACHE classification is strongly related to criteria representative of severity of illness; amount of therapy received and patient outcome. This study will extend validation of APACHE to 4,000 patients; 2,000 consecutive ICU admissions at one institution and 500 admissions to each of four other university hospitals. Validation will be by rigorous sensitivity analysis using multiple regression techniques. The result will be a classification system applicable to a wide range of acutely ill patients. Multi-institutional validation will mean it can be universally applied. The simplicity of its design will encourage dissemination. By describing the severity of illness of various patient groups, APACHE could be an important new tool in the evaluation of intensive care units as well as new medical technologies. Even now it demonstrates early promise of providing new insights into the relationship between intense medical care, the use of technology and improved health. These insights could assist in improving the overall efficacy of medical care.